1. Field of the Invention
This invention relates to a method of ionizing a sample gas, a method of controlling ionization and an ionization apparatus.
2. Description of the Related Art
Though a mass analyzing apparatus (mass analyzing method) is effective for the purpose of analyzing a sample gas, ionization of the sample gas is necessary in order to use this apparatus (method). A typical atmospheric ionization method in wide use at the present time is the APCI (Atmospheric Pressure Chemical Ionization) method, which utilizes corona discharge. This method brings about chemical ionization under atmospheric pressure by spraying a sample solution and simultaneously heating the same to vaporize the solvent, placing a needle-shaped high-voltage electrode in the vaporized solvent and applying a positive or negative high voltage to the electrode to thereby induce corona discharge. With APCI, first the carrier gas that is the main component is ionized by the corona discharge. For example, if air is the main component, N2+ or O2+ is generated. These ions ionize various impurities contained in the sample gas. If steam, an oxygen-containing compound (alcohol, etc.) or a nitrogen-containing component, etc., having a high degree of polarity is present as an impurity gas, then H+ (H2O)n, H+ROH(ROH)n or NH4+(NH3)n ions will eventually be generated. If hydrocarbon compounds or the like are included as impurities, almost no ionization of these takes place. There is need for a method of selectively ionizing difficult-to-ionize components such as hydrocarbons under the coexistence of alcohol or the like.
The Penning ionization method is available as a method that is capable of selectively ionizing a specific component in a sample gas. The principle of the Penning ionization method is described in Kenzo Hiraoka, “Principle and Application of Low-Temperature Plasmas”, Shitsuryo-Bunseki, Vol. 33, No. 5, pp. 271–306 (1985), especially pp. 275–276 “2.2 Penning Ionization”.
In Penning ionization, only atoms and molecules having an ionization energy lower than the metastable energy of a metastable excited species are converted to positive ions selectively. Electrons are released from atoms or molecules in the course of positive ionization. If these electrons recombine with positive ions, however, ionization efficiency declines.